Non-volatile memory technologies are divided in particular into two main categories, EEPROM memory and flash memory. Each category has its advantages and its drawbacks. For example, flash memory has the advantages of having very small memory cells, block granularity, and a fast access time, but the drawbacks of being less durable than EEPROM, of having complex peripheral algorithm circuits, and of consuming a substantial amount of energy for writing. As regards EEPROM memory, in particular page-accessible EEPROM memory, it has in particular the advantages of having high durability, simpler peripheral circuitry, a first access time, page granularity, lower energy consumption for writing, its main drawback being that its memory cells are bulkier.
Consequently, due to the respective sizes of the peripheral circuits and of the memory cells, EEPROM memory is more compact than flash memory below a storage density ceiling. However, this ceiling is continually being raised, due to reductions in the size of electronic components in both technologies.
For example, this ceiling currently corresponds to a density of about 8 to 16 Mb.
It is therefore desirable to decrease the size of EEPROM memory cells further in order to benefit from their advantages in higher density memory.
EEPROM memory cells typically include an access transistor and a state transistor connected in series, the access transistor allowing the state transistor to be coupled to a bit line via its conduction terminals. The state transistor allows a charge representative of a logic datum to be stored in a non-volatile manner in its floating gate, the bit line allowing a memory cell to be selectively accessed for writing or reading.
A writing operation typically comprises an erasure followed by a programming operation. During the erasure, a positive high erasing voltage is applied to the control gate of the state transistor, injecting, via Fowler-Nordheim effect, an electron charge into the floating gate, from the source line, for example connected to ground. During the programming operation, a positive high programming voltage is applied to the drain of the state transistor, via the bit line and through the access transistor, extracting, via Fowler-Nordheim effect, the electron charge potentially stored in the floating gate.
The high programming voltage is for example of the order of 13 to 15 volts and the access transistor must be robust enough to transmit it, which is one cause of the greater bulk of EEPROM memory cells.